1. Field of the Invention
This invention relates to a melt molding material having excellent stress cracking resistance, polar solvent resistance and water resistance, also good gas barrier properties against oxygen etc. and excellent moldability, in particular, extensibility, which comprises a saponified product of a silicon-containing ethylene-vinyl acetate copolymer (hereinafter referred to as EVOH) and have been imparted with a wide range of properties suitable for various processing modes.
2. Description of the Prior Art
EVOH is a useful polymer material having high gas barrier properties and high transparency and also excellent in oil resistance and aroma-retaining properties, and is suitably and widely used in films, sheets, containers etc. However, EVOH lacks flexibility, although having great stiffness, and hence has a disadvantage that mechanical characteristics, for example, impact resistance etc., are low. On the other hand, since EVOH is susceptible to polar solvents, for example, alcohols such as methanol, ethanol etc. and also has great hygroscopicity, there are disadvantages, for example, physical properties such as flexibility, Young's modulus etc. of the molded product greatly fluctuate with the change in atmospheric moisture, temperature etc.; the moisture dependency of gas barrier properties is great and thus the gas barrier properties are reduced under high moisture conditions; further in a use where a molded product is directly contacted with any of the above-mentioned polar solvents, the molded product is swollen with the solvent, and in an extreme case, for example, when sterilization treated with hot water, etc., films, containers etc. undergo deformation, mutual adhesion etc., and so forth. Further, where EVOH molded bottles are contacted with certain types of surface active agents, polar solvents, agricultural chemical liquids etc., there is also a disadvantage that coupled with residual stress generated on molding, stress cracking is often caused.
It is an extremely important subject in practice to improve these various disadvantages. In order to offset said disadvantages, post-modification by a method of conducting a crosslinking operation on a molded product is imparted. For example, as a method of crosslinking plastic films, there is an electron radiating method applied to crosslinking of polyethylene films, but this method requires an expensive device and moreover requires strict control of the production conditions and therefore, with resins such as EVOH which are more easily decomposable and have a narrower molding temperature range as compared with the polyethylene films, there will be considerable difficulty in the materialization of this technique. Further, although various attempts have been made on the modification of EVOH resins for the purpose of improving said various disadvantages, no EVOH which has been uniformly modified without any spots and is free from these various disadvantages has yet been discovered. In other words, main attempts which have hitherto been made were to aim to impart a crosslinked structure to a part of EVOH by a post-modification method, and examples thereof include that which comprises treating EVOH with a boron compound (Japanese Patent Publication No. 20615/1974 Official Gazette), that which comprises reacting an organosilicon compound such as an organosilane compound, an organosiloxane compound etc. in the presence or absence of a peroxide and imparting a crosslinked structure to a part of EVOH in the presence of water or a silanol condensation catalyst (Japanese Patent Application Laid-open No. 20946/1976 Official Gazette, Japanese Patent Application Laid-open No. 53541/1976 Official Gazette, Japanese Patent Application Laid-open No. 20058/1979 Official Gazette etc.) etc. These prior art techniques have been inevitably still unsatisfactory in a respect that the uniformity of the degree of modification of EVOH is to be obtained, more specifically, for example, the localization of the crosslinked structure is to be avoided, seemingly owing to disadvantages due to that a boron compound or an organosilicon compound is added to EVOH to impart a crosslinked structure to a part of said resin, that is, the amount of said crosslinking agent added is considerably small and also due to that said addition, mixing and kneading and, further, reaction to and with the EVOH are by a post-modification method conducted in the conventional manner using various mixers, rolls, extruders, etc., and as a result, in molded products such as films, sheets, containers etc., although some improving effect is observed in various characteristics such as hot water resistance, humidity dependency of gas barrier properties, stress cracking resistance, polar solvent resistance etc., they have a drawback that their effects can not be sufficiently manifested due to the generation of localized spots in the molded products or there is a fluctuation in characteristics from product to product, and therefore, an improved EVOH having higher uniformity has been strongly desired.
Furthermore, Japanese Patent Application Laid-open No. 123189/1975 (corresponding to U.S. Pat. No. 4,097,436) discoloses that vinyl acetate and a vinylalkoxysilane are copolymerized and saponified to obtain a polyvinyl alcohol containing silicon, and that on that occasion, it is possible to replace a part (30-40 wt%) of the vinyl acetate by other components (ethylene etc.), but there is neither description regarding the silicon-containing EVOH having an ethylene content of 25-55 molar % nor description that it may be used as a melt molding material.